philg hates software: Modifying Constants

• From: Phil!Gregory
  Date: 22:28 on 10 May 2004
  Subject: Modifying Constants
  
  A minor thing, really, but I've just wasted several hours on this bug:
  
  I've been working on convincing a Delphi program to call procedures in a
  DLL.  This is new to me, so I figure I'll do things wrong.  Thus, when
  things go wrong, I blame myself, sometimes, as I found, needlessly:
  
  procedure SimLibLogon(pszDBName: PChar) external "EKDWS.DLL";
  
  procedure DoStuff;
    var
      System : String;
  begin
    System := 'VADER';
    SimLibLogon(PChar(System));
  end;
  
  and I get an access violation.  The problem?  Delphi is stricter about
  types than C.  So, with the prototype I wrote for the procedure, Delphi
  decides that the string is a constant, and just calls it with a pointer to
  wherever the program stores its string constants.
  
  The DLL was written in C.  It gets a character pointer which, logically,
  ought to be constant, but isn't strictly so.  It tries to modify it.
  (Consensus here is that it's probably trying to upper-case the string.)
  The string is in non-modifiable memory, so I get an error.  I blame
  myself, try to fix it, and run in circles for a couple of hours.
  
  Solution?
  
    System := Format('VADER', []);
  
  Compiler has been fooled, and the string is now in modifiable memory.
  Sigh.
  
  
  

Generated at 10:28 on 16 Apr 2008 by mariachi